Surface acoustic wave (SAW) devices have been studied and gradually commercialised since the mid 1960s. Such devices typically have electrodes in the form of interlocked “fingers” (so-called inter-digital electrodes) formed on a piezoelectric substrate. When high frequency signals are applied to the input electrodes, mechanical vibrations in the form of travelling acoustic waves are generated in the piezoelectric substrate which are picked up by the output electrodes. Generally speaking, when the wavelength of the surface acoustic waves and the period of the electrode “fingers” are the same, the magnitude of the surface acoustic waves are at their greatest and the device has a low electrical impedance. At other input frequencies, the device appears to have a higher electrical impedance.
Thus, such a so-called “SAW resonator filter” can be made to have a very precise and narrow (typically having a Q factor over 1000), band pass characteristic. Furthermore, since surface acoustic waves travel across the substrate 100000 times more slowly than the speed of electromagnetic waves, such devices are generally compact. In practice, such devices can be used in a ladder configuration (with a plurality of shunt and plurality of series resonator filters used together). This allows a combined band pass characteristic to be tuned as desired.
Thus such devices have found many uses. However, such devices suffer from two significant disadvantages which prevents their use in some applications. Firstly, band pass filters produced using SAW resonators typically have relatively high insertion losses typically of a minimum of 1 or 2 dB. The state of the art presently is an insertion loss of about 1 dB in the pass band with a rejection of about 15 dB in the stop band for a single stage band pass ladder filter. The losses typically occur as a result of visco-elastic attenuations and/or mode conversions from SAW to bulk acoustic waves when the electrical energy is converted to acoustic energy and travels around the SAW filter cavity. Secondly, the power handling capability of SAW filters is limited. At high powers, the ultrasonic vibration to which the metallic electrodes are subjected eventually causes the metal grain boundaries to migrate. Thus, for example, at the present 1800, 1900 and 2100 MHz cellular mobile bands, such filters cannot be used for a mobile handset duplexer because at these frequencies, such filters cannot survive for a realistic length of time at the desired power levels of approximately 30 dBm.
Relatively little work has been done on SAW notch or band reject filters to date. Of the little work which has been reported, most of it has focussed on the development of narrowband notch filters. One of the first publications on SAW notch filters was in U.S. Pat. No. 4,577,168 (Hartman). Various techniques for implementing SAW notch filters are described in which the conductance within the passband of a Single Phase Unidirectional Transducer (SPUDT) SAW transducer was used as an impedance element to create a notch filter. One implementation used the impedance of a SPUDT in conjunction with an RF transformer and other implementations consisted of replacing the capacitors in a bridge—T type notch filter with a SPUDT transducer impedance element. This approach has one disadvantage in that SPUDT transducers fall into the class of Finite Impulse Response Devices and hence the device must be made longer if narrow notch bandwidths are to be achieved. Furthermore, SPUDT type devices are not easily manufactured at elevated frequencies since ⅛ wavelength electrodes are required.
A variation of this technique is described in S. Gopani and B. A. Horine “SAW Waveguide-Coupled Resonator Notch Filter”, Ultrasonics Symposium, 1990, in which a Two-Pole Waveguide Coupled (WGC) Resonator is embedded in an all pass network to implement a notch filter. This technique has two major disadvantages. Firstly the WGC resonator is limited to Quartz hence only bandwidth of 0.1% are attainable and secondly, the resonator has a very poor shape factor of around 5.3 since a typical device might have a 40 dB stopband width of 84 kHz and the 3 dB stopband width of 444 kHz. The device described had a centre frequency of 247 MHz and the insertion losses in the passband were of the order of 4 dB.
A further modification is described in P. A. Lorenz and D. F. Thompson, “Wide Bandwidth Low Cost SAW Notch Filters”, Ultrasonics Symposium, 1998. This technique consisted of placing two single pole SAW resonators in series with a resonator in between them. This technique achieved notch depths of more than 40 dB but had a relatively poor shape factor of 4.3 where the 40 dB stopband width was 86 kHz and the 3 dB stopband width was 370 kHz at a centre frequency of 420 MHz. Insertion losses in the stop band were approximately 5 dB or less.
Other simpler implementations consist of using a single pole SAW resonator in series with the signal to obtain a notch at the anti-resonance frequency. Although simple, this filter has a relatively narrow rejection bandwidth, and the shape factor is very poor.
All the reported SAW notch filter developments focused on narrow band notch filters versus wider band reject filters. Furthermore, the techniques consisted of using the impedance of a SAW SPUDT or resonator device in an all pass network to generate a notch response near the passband of the SAW device and leveraged the capacitive properties of the SAW device away from the notch to form an all pass network. Rather poor notch shape factors and insertion losses have been achieved in the reported literature. Therefore there is a need for wider rejection band devices and or lower insertion losses within the passband.